Ground-based gravitational wave observatories with frequency sensitivities down to a few Hz, like the Einstein Telescope (ET; [1]}) and Cosmic Explorer (CE; [2]} will discover millions of coalescing stellar binary BHs (\(\sim 10^2 M_\odot \) ) out to \(z\sim 10-15\)  (see Figure REF ), thus probing the existence of light seeds that are expected to be to faint for EM facilities (e.g., [3]} and references therein). Complementary, Decihertz Observatories, such as the proposed Deci-hertz Interferometer Gravitational wave Observatory (DECIGO) and the Atom interferometer Observatory and Network (AION), will be sensitive to \(\sim 10-10^4 \, \rm M_\odot \)  binary BHs at \(z>12\)  (e.g., [4]}, and references therein).
